At first, Rosencrantz and Guildenstern rejected black clothes (4)
I believe the answer is:
garb
'clothes' is the definition.
(garb is a kind of clothes)
'at first rosencrantz and guildenstern rejected black' is the wordplay.
'at first' suggests taking the first letters.
'rejected' is a reversal indicator.
'black' becomes 'b' (abbreviation as in pencil classifications).
The first letters of 'rosencrantz and guildenstern' is 'rag'.
'rag' backwards is 'gar'.
'gar'+'b'='GARB'
(Other definitions for garb that I've seen before include "costume" , "Outfit" , "Clothe" , "Style of clothing" , "Raiment" .)